1. Field of the Invention
The present invention relates generally to object inspection apparatus and more particularly to an apparatus and method for inspecting photomasks having duplicate die patterns thereupon for defects by utilizing an edge and corner comparison defect detection technique to identify defects as differences between otherwise duplicate die patterns. Alternatively, die patterns may be compared to corresponding data taken from a data base.
This application is related to the co-pending application of Kenneth Levy, Steven L. Bucholz, William H. Broadbent and Mark J. Wihl entitled "Photomask Inspection Apparatus and Method with Improved Defect Detection", Ser. No. 492,658, filed May 8, 1983 and the application of Don L. Danielson, Mark J. Wihl and David A. Joseph, entitled "Reticle Inspection System", Ser. No. 474,461, filed Mar. 11, 1983, both of which are assigned to the assignee of the present application. The disclosures of both applications are expressly incorporated herein by reference.
2. Description of the Prior Art
Automatic photomask inspection systems have been commerically available for several years now, and examples of such systems may be found in the U.S. Patents to K. Levy and P. Sandland, No. 4,247,203, entitled "Automatic Photomask Inspection System and Apparatus" and No. 4,347,001, also entitled "Automatic Photomask Inspection System and Apparatus". The devices disclosed in these patents optically project images of two die patterns onto arrays of photosensors which are scanned periodically as the photomask is moved relative thereto. The scanned or sampled images are then digitized to form two dimensional pixel representations of the die pattern which are stored in digital form and processed by defect detection circuitry to locate and identify defects. Defect detection is normally accomplished by comparing the digitized images of two similar die patterns and noting any differences between the two; such differences being identified as defects.
A significant problem inherent in most prior art inspection systems is that the efficiency of the defect detection circuitry is relatively poor for defects located near corners. In order to overcome such problems, some kind of template matching technique is usually employed. However, in using such techniques, there are almost always cases involving unusual quantization in which errors occuring at or near image corners are missed.